A-LevelChemistryGroup 29701/42 · Oct/Nov 2023 · Q56 Marks
Some of the ionic compounds of Group 2 elements undergo thermal decomposition. Thermal decomposition of solid anhydrous magnesium ethanedioate, MgC2O4, occurs above 650°C. The products are magnesium oxide and a mixture of two different gases, one of which gives a white precipitate with saturated calcium hydroxide solution.
- (a)Complete the equation for the thermal decomposition of MgC2O4. MgC2O4→[1]
- (b)Suggest which of MgC2O4 or CaC2O4 undergoes thermal decomposition at a lower temperature. Explain your answer.[2]
- (c)The ethanedioate ion is oxidised by acidified KMnO4· 5C2O42- + 2MnO4¯ + 16H+ → 10CO2 + 2Mn2+ + 8H2O An experiment is performed to find the solubility of MgC2O4 in water. A 40.0cm³ sample of saturated aqueous MgC2O4 requires 27.05 cm³ of 0.00200 moldm-3 acidified KMnO4 to oxidise all the C2O42– ions. Calculate the solubility, in moldm³, of MgC2O4 in water. Show your working. solubility = ............................ moldm-³[3]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2023). The mark or mark type is shown in brackets.
- [1] (a) MgC2O4 → MgO + CO2 + CO
- [1] (b) magnesium ethanedioate decomposes at lower T because Mg2+ has smaller radius than Ca2+
- [1] (b) so anion is more polarised by Mg2+
- [1] (c) (27.05 ÷ 1000) × 0.02 = 5.41 × 10–5 moles MnO4
- [1] (c) 5.41 × 105 × 5/2 = 1.3525 × 10–4 moles C2O42-
- [1] (c) 0.00338 mol dm-³
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